This invention relates to the control of nuclear reactors and particularly to the control of the axial power distribution in pressurized water power reactors.
A variety of control rod types, each typically containing a high thermal neutron absorber such as B.sub.4 C, are used to provide many of the power-related control functions necessary for satisfactory reactor performance. Satisfactory performance requires the ability to quickly shut down, or "scram", the reactor at the onset of a potentially dangerous condition, so that the neutron power generated may be quickly reduced to substantially zero. A second form of control permits the power output of the reactor to be continuously varied between zero and the full power rating of the system. Finally, the power shape or distribution within the core is controlled to avoid localized hot spots which are a safety-related limitation on the permitted power output of the reactor.
Conventionally, a plurality of control rods are ganged to remain out of the core during normal operation, ready to be scrammed simultaneously in the event of a dangerous condition. These are called shutdown rods and are not used during normal operation. The power level changes during normal operation are performed by regulating rods which are typically divided into four or five groups, each sequentially inserted into or withdrawn from the reactor core as the power level is to be lowered or raised. Typically the first regulating group, which may consist of four to eight control rods symetrically disposed about the reactor core, will enter the core region from the top and continue inserting to about 60% of the full axial extent of the core at which time the second regulating group will enter the core. This staggering or overlap of the regulating rods is typically fixed between each group and does not vary over the course of the operating history of the reactor. A third set of control rods, the part length rods, contains poison material only over a part of each rod. The part length rods are inserted into the core and moved therein for the purpose of controlling the axial power shape.
Control of the axial power shape is important during both full and intermediate power operation. Accordingly, the part length rods are moved independently of the regulating groups to control the axial shape while the power level may be maintained constant or, if desired, adjusted using the regulating groups. Another known technique for adjusting the axial power shape while maintaining constant power level is to change the soluble boron concentration in the moderator in order to influence the moderator temperature coefficient of the reactor. The moderator temperature coefficient has a predictable effect on the axial power shape. This technique, however, requires significant adjustment of the regulating groups because the change in boron concentration has a significant effect on core reactivity. Furthermore, changing the boron concentration requires operation of boron injection and dilution systems which produces undesirable radioactive waste products.